The present invention relates to a method and an apparatus for cleaning substrates, especially semiconductor wafers, according to which prior to and/or during the cleaning process, a fluid is applied to the substrate.
Such an apparatus is known, for example, from WO 96/10463. With this apparatus, a water film is applied to the substrate surface that is to be cleaned, and simultaneously or subsequently steam is conveyed onto the surface that is to be cleaned in a directed manner via a nozzle. The steam rapidly cools in the water film, whereby small steam bubbles implode and thereby generate a pulse in the water film that loosens the particles from the surface that is to be cleaned.
With this known apparatus, particles that strongly adhere to the surface that is to be cleaned can remain if the pulse is not sufficient to dislodge such particles. Furthermore, the apparatus requires a steam generator that increases the overall size of the apparatus. In addition, the steam comes into contact with metal within the steam generator and in the subsequently arranged components that convey media, so that there exists a danger of contamination of the substrate that is to be cleaned with metal ions.
Reference is also made to U.S. Pat. No. 5,651,834, which discloses a method for cleaning printed circuit boards. With this method, a thin water film is applied to a side of the circuit board that is to be cleaned. Subsequently, the side of the circuit board that is to be cleaned is bombarded with ice crystals that pass through the water film and strike the circuit board surface in order to clean the same. The water film has the function of diverting electrical charges that occur during the bombardment with ice crystals so that these charges do not damage the printed circuit boards.
Proceeding from the known apparatus, it is an object of the present invention to provide an apparatus and a method which, in a straightforward and economical manner, permit a reliable cleaning of substrates without the danger of damaging the same.
The object is inventively realized with a method for cleaning substrates, especially semiconductor wafers, according to which prior to and/or during the cleaning process a fluid is applied to the substrate, and ice crystals are introduced into the fluid located on the substrate, in that the depth of penetration of the ice crystals into the fluid is controlled in order to achieve a precise pulse generation and hence a precise cleaning. By controlling the depth of penetration of the ice crystals, it is furthermore possible to prevent the ice crystals from striking the substrate surface that is to be cleaned and thereby damaging such surface, since prior to striking the surface the ice crystals change into the gaseous state. Due to the large temperature difference between the ice crystals and the fluid, the ice crystals explosively change into the gaseous state. The thereby generated pulse, which is considerably greater than the pulse generated by the implosion of small steam bubbles, dislodges particles from the substrate surface that is to be cleaned. To carry out the inventive method, it is merely necessary to have a fluid feed line as well as a feed line for a gas that is under high pressure, so that the apparatus required for carrying out the method has a small size. In particular, for the cleaning of semiconductor wafers, which is generally effected in clean rooms, it is important to the extent possible to use apparatus having small dimensions, since clean rooms generate considerable cost not only during the manufacture but also during operation. Since it is necessary to provide only a feed line for the fluid and for a gas that is under high pressure, the components that convey the media can be made without metal, so that the danger of contaminating the substrate with metal ions can be precluded. Furthermore, the size and temperature of the ice crystals, and hence the magnitude of the pulse that is generated when the crystals change into the gaseous state, are easy to control, as a result of which on the whole a good control of the cleaning process results.
In order to control the magnitude of the pulse that reaches the surface of the substrate, the thickness of a fluid layer on the substrate and/or the temperature of the fluid are advantageously controlled. As a consequence of the thickness of the fluid layer, the distance between the source of the pulse and the surface of the substrate, and hence the strength of the pulse at the substrate surface, can be set. By controlling the fluid temperature, the intensity of the change of the ice crystals into the gaseous state, and hence the magnitude of the pulse, can be varied.
Pursuant to a further specific embodiment of the present invention, the size and/or temperature of the ice crystals are controlled, both of which determine the force of the pulse that is generated during the change of the ice crystals into the gaseous state.
Pursuant to a preferred specific embodiment of the present invention, the ice crystals are preferably directed at an angle onto the substrate in order to prescribe a direction of cleaning. The ice crystals are advantageously produced in at least one ice nozzle that is directed against the substrate. In so doing, during the cleaning a relative movement is advantageously produced between the ice nozzle and the substrate in order to ensure a cleaning of the entire substrate surface. Pursuant to one specific embodiment, the substrate and/or the ice nozzle is moved linearly. The substrate is advantageously furthermore rotated, so that loosened particles are moved out of the region of the wafer by the centrifugal force that results due to the rotation.
Pursuant to a further specific embodiment of the present invention, the substrate is inclined relative to the horizontal during the cleaning process in order to ensure that the fluid film and the loosened particles constantly flow off.
The fluid is advantageously deionized water, and the ice crystals are preferably CO2, since CO2 sublimates at normal pressures.
The object of the present invention is realized in an apparatus for cleaning substrates, especially semiconductor wafers, that includes a device for applying a fluid to the substrate, by a device for introducing the ice crystals into the fluid. The already aforementioned advantages are achieved with such an apparatus.
The apparatus advantageously has at least one nozzle for applying the fluid, with such nozzle preferably being embodied as a wide slot nozzle having a width that is greater than or the same as the width of the substrate. By using such a wide slot nozzle, it is possible in a straightforward manner to ensure that the entire substrate surface that is to be cleaned is covered with the fluid. The apparatus preferably also has a device for tempering the fluid, as a result of which the already aforementioned advantages are achieved.